1. Field of the Invention
This invention relates to the range art and particularly to the use of temperature sensors to monitor the temperature of a glass-ceramic cooktop surface and to control power to the heating units of a glass-ceramic cooktop surface.
2. Description of the Prior Art
In order to improve the cleanability of cooktops of domestic ranges as well as built-in counter cooktops, the standard porcelain enamel cooktop surface with separate electrical heating elements or gas burners has been replaced in certain models of appliances by high resistivity continuous glass-ceramic plates, which are heated by either electric or gas heating units disposed beneath the cooktop surface of the continuous glass ceramic plate. Such plates are of generally milk-white, opaque, glass-ceramic or cystalline glass material sold under trademarks as "PYROCERAM", "CER-VIT", and "HERCUVIT". This glass-ceramic material has a low thermal expansion coefficient, and it has a smooth top surface of almost polished glass finish or texture that presents a pleasing appearance and is also readily cleanable. The smooth continuous surface prevents the drainage of spillovers underneath the cooktop in the area of the heating elements.
It has been long known that the electrical resistance of the glass-ceramic material used for such cooktops decreases in a predictable manner with increasing temperature. However, advantages of this characteristic has not heretofore been taken in the manner of the present invention.
Such smooth surface glass-ceramic cooktops have found wide acceptance and have become very popular for use on both electric and gas ranges of a variety of types.
For example, in U.S. Pat. No. 3,710,076 to Frazier, there is disclosed a single-plate, utensil-supporting, glass-ceramic cooktop having a plurality of open coil heating elements positioned therebeneath to provide several areas for surface cooking. In this system, the main mode of heat transfer between the open coil heater and the glass-ceramic cooktop is by radiation since the heater is vertically spaced from the cooktop by an air gap. In order to produce high radiant heat to obtain acceptable heating rates, the heater coil is operated at relatively high temperatures on the order of 1800.degree. to 2000.degree. F. at a wattage rating of about 2000 watts at 236 volts AC. Because of the high operating temperatures, it is essential to monitor the temperature of cooktop and control heater power to optimize thermal and cooking efficiency.
U.S. Pat. No. 3,710,076 to Frazier additionally discloses a temperature sensor for sensing the temperature of the surface to provide a control signal for adjusting the heat radiation by an electrical resistance heating element. In the aforementioned Frazier patent, a divider extends centrally across the interior of the container or box-like cooking unit and the temperature sensor has at least one flat side supported in a channel provided in the upper surface of the divider with the flat side of the sensor in firm physical contact with the underside of the cooktop.
Such sensing units suffer from several disadvantages. For example, response time is slow because the sensor is positioned to the side of the heated area and away from the region of maximum glass temperature. This is so because the glass-ceramic material has a high thermal mass, thus a slow response requiring a longer time to heat up and cool down. Further, the heat is stored in the glass-ceramic plate as well as in the insulating support block or pad for the heating element and, because the region of maximum glass temperature is directly above the heating elements, the sensor does not reflect a true reading of the temperature of the glass at the cooking areas.
When open coil heaters are used at a spaced distance below the plate there is also a poor thermal coupling between the heater and the glass-ceramic plate. In order to transfer the heat from an open coil heater to the glass-ceramic plate, the heater has to operate at high temperature which creates several problems such as poor efficiency of the system, high heat losses, overheating of components, and high cooktop temperatures. Glass-ceramic cooktops and surface units with open coil heaters also present a safety hazard in the event the glass-ceramic plate is broken.
In an attempt to overcome some of the aforenoted disadvantages of open coil heater assemblies, resort has been made to the use of various forms of heating arrangements. For example, in U.S. Pat. Nos. 3,612,827 and 3,632,983 to Dills, which are assigned to the assignee of the present invention, there is shown a glass-ceramic cooktop including a shallow mounting or rough-in box that contains a filler plate that has recesses for accommodating the heating units and wiring raceways for containing the electrical lead wires. The heating units are constructed with at least one flat side positioned directly against the underside of the cooking area of the flat cooktop surface.
To further improve the efficiency of the heating system, resort has been made to use of film heaters. Typical examples of glass-ceramic cooktops using film heaters are shown in the Hurko U.S. Pat. Nos. 3,067,315 and 3,883,719, both assigned to the assignee of the present invention. Such film heaters are of serpentine shape, and they are bonded directly to the plate. They provide a most efficiency heating system for glass-ceramic surface heating units or cooktops because the film strips have a very low thermal mass and good thermal coupling with the plate, resulting in quicker response to heat-up and cool-down conditions. The film heater stores very little heat, and it radiates very little heat in a downward direction because of its low emissivity surface. One disadvantage of film heater designs for solid plate surface heating units is the relatively high cost of film materials because they are of noble metals, such as gold and platinum and as a consequence thereof, resort may be made to etched foil heaters as shown in U.S. Pat. No. 4,032,750 to Hurko, also assigned to the assignee of the present invention.
It is extremely difficult to maintain an even temperature distribution across a glass-ceramic plate when heated directly by an open coil resistance heating element or a metal sheathed resistance heating element of looped configuration. Heat diffuses very slowly laterally through the glass-ceramic material, and hence, hot spots may be created on the plate surface nearest the areas of contact between the heater and the glass, as well as between the glass and the bottom of a cooking utensil, particularly if the cooking utensil has a warped or uneven surface. This type of glass cannot exceed an operational temperature of about 1300.degree. F. at any point, hence, the total heat output of a glass ceramic surface heating unit would be reduced if the plate is provided with an uneven temperature distribution. In the absence of a temperature-limiting means, the plate would have to be underheated in order to avoid damaging the glass-ceramic plate.
To provide for a more even temperature distribution across the glass-ceramic plate, resort may be made to the use of a glass-ceramic plate surface heating unit having a high thermal conductivity layer such as aluminum or copper cast in a recess formed on the underside of the plate. A metal sheathed electrical resistance heating element with an underlying reinforcing member is cast into the high conductivity layer, so that the layer serves both as a mechanical and thermal coupling means between the heating element and the plate as well as a heat spreader means across the plate. The underside of the plate includes a plurality of cavities so as to increase the area of contact between the high conductivity layer and the glass-ceramic plate. Such an arrangement is shown in U.S. Pat. No. 3,885,128 to Dills, assigned to the assignee of the present invention.
Regardless of the type of heating unit employed, it is important to limit the operating temperature of the glass-ceramic plate to a temperature below about 1300.degree. F. This can be done by introducing a temperature-limiting means in the solid plate surface heating element unit such that the power to the heating element is cut off if the temperature of the surface rises to a predetermined temperature.
In the aforementioned Dills U.S. Pat. No. 3,885,128, the temperature limiting means comprises a temperature sensor in the form of an elongated bulb which is positioned outside the outermost coil of the heating element and is positioned on the reinforcing framework and cast in the heat spreader layer. This sensor is a bulb-like member that is filled with a high temperature thermostatic fluid such as sodium potasium (NaK) or the like. The sensor communicates with a temperature responder by means of a capillary tube. This temperature responder is a single-point, temperature-limiting switch or thermostat that is set at a critical temperature of about 1250.degree. F. This temperature responder includes switch means in a series circuit with the heating element such that if the critical temperature of the heat spreader casting would be reached the power circuit to the heating element would be broken and the heating element de-energized. A similar arrangement is utilized in the Hurko U.S. Pat. Nos. 3,622,754 and 3,883,719. Such sensors suffer from the disadvantage of delayed response and, because of positioning, being unable to respond instantaneously to changes in temperature of the heated area of the glass-ceramic plate surface of the cooktop.
It should also be noted that cooking utensils absorb heat from the cooking surface of the glass and this absorption depends to a great extent on the fit, i.e. the amount of contact betwen the cooking surface and the bottom of the cooking utensil or pan. Good fitting pans readily absorb heat so that the heating unit may operate a high percentage of time at a given temperature limit. Poor fitting pans, on the other hand, because of poor absorption rates, require that the system reduce its total heat input to even less than the no load value, and this response must be almost instantaneous.
Accordingly, it is important for a temperature sensor to measure the glass temperature at the interface between the underside of the glass-ceramic plate and the heater assembly, i.e. the region of maximum glass temperature.
The principal object of the present invention is to provide a temperature sensor for a solid, glass-ceramic plate surface heating unit which has rapid response, is simple to manufacture, low in cost, and which provides a high performance system.
Another object of the present invention is to provide an improved glass-ceramic plate cooktop having a temperature sensor disposed to monitor the cooktop at the region of maximum temperature.
A further object of the invention is to provide a temperature sensor particularly suitable for use as a temperature limit sensor for a glass-ceramic cooktop.
A still further object of the invention is to provide a temperature sensor for a glass-ceramic cooktop which may be used in combination with a variety of particular heating elements.
It is another object of the invention to provide a glass-ceramic cooktop temperature sensor which includes fail-safe protection against an open circuit in the sensor circuit.